Meaning
Reactive chemical intermediates are formed when a hydrogen atom is abstracted from a branched carbon atom along a polymer backbone. The stability of a tertiary carbon radical makes polymers like polypropylene highly susceptible to oxidative degradation during high temperature moulding. This intermediate is formed more easily than primary or secondary radicals because the surrounding alkyl groups stabilize the unpaired electron.
Preventing this reaction requires the addition of primary antioxidants that can quickly donate a hydrogen atom to cap the radical before chain scission occurs.
Polymer Oxidation
Heat and shear break the carbon hydrogen bonds to initiate the degradation process. Once formed, the tertiary carbon radical reacts rapidly with oxygen to create a peroxy radical, which then abstracts hydrogen from a neighboring chain. This self sustaining cycle leads to rapid molecular weight reduction and brittle moulded parts.
Radical Stabilization
Molecular substituents bonded to the radical center share the electron deficiency through hyperconjugation. This stabilization lowers the activation energy required to break the C H bond, making the polymer chain inherently vulnerable to attack. Understanding this vulnerability is key to designing stabilizers for branched polyolefins.
Antioxidant Inhibition
Hindered phenols and phosphite stabilizers are compounded into the resin to interrupt this degradation pathway. These additives act as sacrificial donors, neutralizing the radical intermediates before they can attack the polymer chain.
